ICMCTF2011 Session A1-2: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

Wednesday, May 4, 2011 1:30 PM in Room Sunrise
Wednesday Afternoon

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Start Invited? Item
1:30 PM A1-2-1 Effect of Increased Water Vapor Levels on TBC Lifetime
Aurelie VandePut (CIRIMAT - ENSIACET Toulouse); Bruce Pint, Allen Haynes (Oak Ridge National Laboratory); Ying Zhang (Tennessee Technological University)

While the exhaust in natural gas-fired land based turbines contains 10-15% water vapor, the exhaust with coal-derived fuels or innovative turbine concepts for more efficient carbon capture may be 30-85%. To investigate the effect of increased water vapor levels on thermal barrier coating (TBC) lifetime, furnace cycling tests were performed at 1150°C in air with 10, 50 and 90 vol.% water vapor. The specimens were all from the same batch of a commercial second generation superalloy and had Pt diffusion or Pt-modified aluminide bond coatings and commercially vapor-deposited yttria-stabilized zirconia top coats. The lifetime results will be compared to a prior study where similar coatings were thermally cycled in dry oxygen.

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Research sponsored by the U. S. Department of Energy, Office of Fossil Energy, Coal and Power R&D.

1:50 PM A1-2-2 Oxidation Resistance of Nanocomposite CrAlSiN under Long-Time Heat Treatment
Hsien-Wei Chen, Yu-Chen Chan (National Tsing Hua University, Taiwan); Jyh-Wei Lee (Mingchi University of Technology, Taiwan); Jenq-Gong Duh (National Tsing Hua University, Taiwan)
As a protective coating, the nanocomposite CrAlSiN reveals better mechanical properties and higher thermal abilities than conventional CrAlN coatings. To investigate the oxidation behavior and structure stability, Si was doped into CrAlN films deposited on silicon wafers and sapphire substrates by RF magnetron sputtering and annealed at temperatures ranging from 1000 to 1200oC for 100 hours. The X-ray diffraction patterns revealed that the grain size of as-deposited CrAlSixN (x = 0-12 at. %) coatings became finer with doping silicon. According to SEM micrographs, the growth of oxide layer was retarded with increasing silicon content after heat treatment in air. Additionally, the surface roughness of CrAlSiN using AFM analysis increased slightly even though annealed for a long time. As observed by TEM, the CrAlSiN coatings could well retain the nanocomposites structure after heat treatment at elevated temperature, indicating that CrAlSiN exhibits high structure stability at high temperature. Moreover, the thermal properties of coatings were also analyzed by TGA and DSC. To conclude, doping certain Si content could reduce the grain size and prolong the diffusion paths in CrAlN coatings, thereby effectively inhibiting nitrogen outside diffusion and oxygen penetrate into the coatings. Further, there was no significant variation in the microstructure of CrAlSiN after heat treatment, suggesting that the nanocomposites could preserve the oxidation resistance at elevated temperature.
2:10 PM A1-2-3 HRTEM Study of Arc-Sputtered Nanocomposite TiSiN Thin Films
Jessica Mooney, Efstathios Meletis (University of Texas at Arlington); Yuhang Cheng (American Eagle Instruments, Inc.)
Hardened nanocomposite thin film coatings have been the subject of much interest in the research community because of their high potential hardness (~100 GPa) and oxidation resistance. Much work has shown that the proper incorporation of Si into transition metal nitride coatings traditionally used in tribological coatings and diffusion barriers such as TiN can yield a nanocomposite material consisting of both nanocrystalline and amorphous phases. In order to assess the effects of Si content on material properties, large area filtered arc deposition was used to deposit thin film coatings of TiSiN. (1,2) The Si content of the coatings was varied by using TiSi targets with different Si content. Samples with the lowest and highest concentrations of Si (0.5 at% and 8.0 at%) were the subject of a detailed high resolution transmission electron microscopy study to advance our understanding of the relationship between film microstructure and mechanical properties.

1. Cheng, et al. J Phys D: Appl. Phys. 42 (2009)1.

2. Cheng, et al., Surf. Coat. Technol. 204(2010)2123.

2:30 PM Invited A1-2-4 Moisture-Induced Desktop Spallation of TBCs
James Smialek (NASA Glenn Research Center)

Delayed failure of TBCs is a widely observed phenomenon, even though many of the occurrences are anecdotal and unreported. The phenomenon is characterized by survival of a TBC after considerable thermal cycling and full cooling to room temperature. Yet, after some extended time under ambient conditions, the TBC may be found to be nearly fully delaminated. This has been termed “the weekend effect” or “desktop spallation.” It is generally assumed to result from exposure to ambient humidity, which may be considerably increased compared to the furnace environment. To further demonstrate the humidity factor, oxidized TBCs were subject to water immersion or a water drop exposure after being retained and fully cooled from a thermal cycling treatment. It is often found to then fail quite dramatically in less than a second, after some 10s of seconds of incubation time. To this end, digital video recordings have become a most useful observation technique. We report on early “no bond coat” plasma sprayed coatings, where 1100-1150oC cyclic life and resistance to water immersion are seen to increase as the sulfur content of the superalloy substrate was reduced. EB-PVD coatings using a Pt-aluminide bond coat are also seen to fail by the water drop test after 1150oC cyclic furnace tests. More recently, water drop failure is observed for a commercial TBC coated turbine blade, after oxidation at 1150-1200oC. In support of this phenomenon, colleagues from DECHMA (Rudolphi, Renusch, Schütze) and CNRS Toulouse/SNECMA (Déneux, Cadoret, Hervier, Monceau) have both produced very compelling and informative studies. They have used video, acoustic emission, and extended dry box storage of oxidized TBCs.

The phenomenon is rooted in moisture-induced delayed spallation of the alumina scale formed on the bond coat. In that regard, many studies show susceptibility of alumina scales to moisture, as long as high strain energy and a partially exposed interface exist. The latter result from severe cyclic oxidation conditions, producing a highly stressed and partially damaged scale. In one model, it has been proposed that moisture reacts with aluminum in the bond coat to release hydrogen atoms that ‘embrittle’ the interface. A negative synergistic effect with interfacial sulfur is also invoked.

3:10 PM A1-2-6 A Single Step Process to Form an In-Situ Oxidized Alumina Foam Coating for Alloys for Extreme Environments at High Temperatures
Xabier Montero, Mathias Galetz, Michael Schütze (Dechema e.V., Germany)

A new approach to manufacture a complete thermal barrier system in a single step is being studied during the European FP7 project called PARTICOAT. Spherical Al particles are deposited by screen-printing and air brush on IN738, Rene80 and CM247 nickel based alloys. During the sintering process in air the Al particles are partially oxidized and converted into hollow alumina spheres forming a ceramic “foam” (top coat), and simultaneously an Al rich diffusion layer (bond coat) is formed in the subsurface zone of the substrate.

The coatings were isothermally exposed at 800 and 1000°C in air for up to 1000 hours. The oxide formation and the microstructure of the coating were studied by thermo gravimetrical analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX).

The coatings were adherent for all the tested substrates and temperatures. The CM247 alloy shows the lowest mass gain whereas IN738 and Rene80 show higher mass gains at the tested temperatures. The use of reactive element oxides in the coating has also been tested, showing an improved bond-coat microstructure in the case of HfO2. These results demonstrate the flexibility and viability of this low cost coating concept.

3:30 PM A1-2-7 Oxidation Behavior of Slurry Aluminide Coatings on Stainless Steel Alloy CF8C-Plus
J. Allen Haynes, Beth Armstrong, Sebastien Dryepondt (Oak Ridge National Laboratory); Ying Zhang (Tennessee Technological University)
A new, cast austenitic stainless steel, CF8C-Plus, has been developed for a wide range of high temperature applications, including diesel exhaust components and turbine casings. CF8C-Plus offers significant improvements in creep rupture life and creep rupture strength over standard CF8C steel, with creep strength close to that of Ni-base superalloy 617. However, at higher temperatures and in more aggressive environments an oxidation-resistant protective coating will be necessary. This preliminary study compared the oxidation behavior of CF8C-Plus and aluminide-coated CF8C-Plus under various conditions, including 800oC in 10% water vapor plus air. Due to their economic viability, slurry aluminides were the primary coating system of interest, but chemical vapor deposition and pack cementation aluminide coatings were also compared. Substantial short-term improvements in oxidation behavior were achieved with each type of aluminide coating. However, as temperature and environmental aggressiveness increased, each coating displayed design challenges that will have to be overcome in order to develop economical coatings that are stable and protective on austenitic steel for the desired component lifetimes.
3:50 PM A1-2-8 Nitriding and Coating of a Stainless Steel for Corrosion Protection in Carburizing Atmospheres
Dulce Melo (TRAMES SA de CV, Mexico); Olimpia Salas, Joaquin Oseguera (ITESM, Mexico); Ricardo Torres (Pontifícia Universidade Católica do Parana, Brazil); Roberto Souza (University of Sao Paulo, Brazil)
304 steel is commonly used in applications at high temperatures in carburizing atmospheres. The natural Cr oxide layer that forms on these steels does not provide sufficient protection when they are exposed to those conditions. In the present work, substrates of 304 steel have been first plasma nitrided and then coated with additional Cr oxide by in order to provide a surface that will stand carburization at high temperature. The structure of the nitrided and nitride+coated substrates was characterized by optical microscopy, scanning electron microscopy + x-ray microanalysis, and x-ray diffraction. The results indicate that, despite the difficulty in nitriding materials such as this, that have a passivated surface layer, the advantages of the plasma process allowed the formation of a nitride layer which included the expanded austenite phase. Coating of the nitrided substrates was carried out by reactive magnetron sputtering varying the applied power, the oxygen flow and the application of a bias voltage. The mechanical properties of the coated substrates were investigated by nanoindentation experiments and the adhesion by scratch testing. The response of the coated nitride 304 samples to carburization was evaluated through thermogravimetry in a CH4 atmosphere at 800°C.
Time Period WeA Sessions | Abstract Timeline | Topic A Sessions | Time Periods | Topics | ICMCTF2011 Schedule